Robert John Interval Jr., 37, was being held in a Maricopa County jail. Bond was set at $1 million at his initial court appearance Thursday night.

Interval was arrested on Thursday after detectives said they developed probable cause to implicate Interval in Mustafa's disappearance, according to police spokesman Sgt. Jonathan Howard.

Howard said Interval was arrested without incident in Peoria on Thursday.

"The family is tremendously relieved to know that Robert Interval is in custody and has been charged with the murder of Christine," said Erin Neathery, a spokeswoman for the family. "They've been waiting for this since the first day she went missing."

Court documents obtained by The Arizona Republic on Thursday showed the Phoenix Police Department requested a court order for Interval to unlock his cellphone as part of the investigation.

Mustafa's body has not been found, Howard said, and the investigation is continuing.

Woman planned to leave boyfriend

Christine Mustafa, of Phoenix, has been missing since May 10, 2017. Her family says it's unlike her to leave without checking in on her children, ages 15 and 8 months.(Photo: Phoenix Police Department)

Mustafa, 34, was last seen on May 10 as she was leaving work at a Walgreens pharmacy and failed to show up to her shift on May 11.

Court documents said Mustafa hadn't missed a scheduled day of work in 11 years.

Her keys, cellphone and purse were all left behind after she disappeared, police said.

Mustafa's family said they last spoke to her on May 10 at about 3 p.m., according to court documents.

The family told officials that "Christine had expressed a desire to leave her boyfriend, Robert Interval Jr., because she was fearful of him," and that "Robert had previously made threatening statements that he would kill Christine," according to court documents.

Mustafa's family said she had been making plans to move out of her home and get an order of protection against Interval, court documents said.

Co-workers told officers that Interval had called Mustafa while she was at work about 1 p.m. on May 10, accusing her of cheating on him and threatening to kill her, according to court documents.

Mustafa was last seen by co-workers as she was leaving the Walgreens at about 1:30 p.m on May 10.

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Disappearance kept 'quiet' from daughter

Mustafa had an 8-month-old daughter with Interval and a 15-year-old daughter who is with her father visiting family in Jordan, family members previously told The Arizona Republic.

Neathery said the teenager went to the Middle East on May 12 and hadn't been told of her mother's disappearance beforehand.

Neathery said the family thought it was best to not alert the teenager to her mother's disappearance because they had limited information about the case at the time.

"The family made a group decision that it was best to keep it quiet until it became unviable to do so," Neathery said.

The teen was "devastated" upon being told of her mother's disappearance while she was overseas, Neathery said, adding that it's unclear when she will return.

The 8-month-old was at Mustafa's home with Interval at the time of Mustafa's disappearance, family members said.

Neathery said the baby is currently in the custody of the state Department of Child Safety.

Police find woman's phone, wallet

Police were notified of Mustafa's disappearance and used a lock-pick to enter her home on May 11, court documents said.

They noted Mustafa's car parked in the driveway, her purse (including a wallet and Social Security card) and her cellphone lying on the bathroom floor between the toilet and bathtub.

Court documents said Interval arrived at the residence as officers were investigating and told them that Mustafa had left for work that morning on foot because her vehicle had a flat tire.

Officers noted that Mustafa's Nissan Cube was parked in the driveway and did not appear to have any flat or damaged tires, court documents said.

Interval told officers that he and Mustafa had gotten into an argument on the evening of May 10 and the morning of May 11 over his beliefs that she was cheating on him, but denied that any physical violence occurred during the argument, court documents said.

Police entered a missing-person report for Mustafa after the incident.

Mustafa's friend told police officials she saw a Facebook post from Interval on May 11, advertising that he was selling a Nissan Cube vehicle, a bed, a mattress and his work trailer online.

On May 12, officers and DCS investigators were redispatched to the home to do a welfare check on Mustafa's 8-month-old daughter. Officers made contact with Interval at the front door, noting that he was carrying a loaded .40-caliber handgun, court documents said.

When officers entered the residence, Interval told police that he needed to clean the front room and entry area of his house because his dog had defecated and urinated on the floor, documents said.

Interval showed investigators his daughter's room, saying she had bed bugs and that he had used a cleaning solution to get them out of the mattress and linen.

Authorities removed the baby from the home and issued Interval a temporary custody notice.

Suspect's sister concerned

Court documents said police officials also spoke with Interval's sister, who said that she had flown to Arizona from Ohio around May 13 to spend time with her brother after Mustafa's disappearance.

His sister described Interval to investigators as being "paranoid" and making "concerning" statements, which included him saying, "I took it too far," and "I can never un-see the things I saw," according to court documents.

His sister's account said that Interval told her that Mustafa threatened to take the couple's baby and leave him on May 10, court documents said.

Interval told his sister that Mustafa attacked him, only saying afterward that he "took it too far."

Interval told his sister he wanted to turn himself in only after the child-custody case was completed, but didn't say what he would be turning himself in for, court documents said.

Police officials issued a search warrant of Mustafa's home on May 15, during which investigators noted pillowcases and bedsheets containing a small amount of "possible blood-stained material," according to court documents.

Luminol sprayed on the walls as part of the investigation had a reaction that indicated the presence of blood or other biological material, court documents said.

Interval was detained by a search warrant on May 16 upon arrival at his father's residence, court documents said.

Police officials noted he had the .40-caliber handgun and his cellphone at the time of his detainment, both of which were kept as part of the investigation.

Family grieving, seeking justice

Neathery said the last four weeks have been a "very stressful and emotional period" for Mustafa's family, adding that the family is currently observing Ramadan, a month of fasting for Muslims.

The family is trying to come to terms with Mustafa's disappearance while continuing to mourn the loss of their sister who died of cancer in December, Neathery said.

"They haven't even properly grieved for her, so it's important for them to take time to process this and gather strength to move into the next phase, which is finding Christine's body and bringing her home so she can be properly laid to rest," Neathery said.

Neathery said Mustafa's friends and family as well as volunteers are still organizing Valleywide searches to locate her body.

"There's a fierce group of family and friends who won't be played," Neathery said. "We're going to find her and make sure he is brought to justice."